With the aggressive growth of battery powered portable electronics (e.g., cell phones), primary-side feedback controlled AC/DC converter has replaced the traditional secondary-side feedback controlled AC/DC converter in switching mode power supply field with small power due to its advantages such as low cost and compact size.
FIG. 1 illustrates a typical circuit diagram of an exemplary prior-art primary-side feedback controlled AC/DC converter comprising a transformer 10, a primary switch 20, a secondary rectifier 40, an output capacitor 45 and a controller 70. Controller 70 supplies powers from the auxiliary winding of the transformer 10 via a rectifying diode 60. The output voltage is feed back to the controller 70 via the auxiliary winding of the transformer 10, and then sensed by voltage divider resistor R2 and R3 at FB node. A resistor 30 senses the current flowing through primary switch 20.
FIG. 2 illustrates the operation principle of the prior-art primary-side feedback controlled AC/DC converter shown in FIG. 1. At the beginning of each cycle, the controller 70 turns the primary switch 20 on. During the on time, the primary winding current Ip in the inductor of the primary winding is increasing with time at a defined positive slope. While the increase of primary winding current Ip enables the voltage Vcs to hit a reference value VREF1 of the comparator built in the controller 70, the primary switch 20 will be turned off and then a secondary current IS in the output winding starting with a certain peak value is decreasing until reaching zero. When the secondary current IS decreases to a defined value (such as one third of the peak value), the controller 70 samples the primary-side input voltage VAUX of the transformer 10, then the output voltage can be expressed as:
                                                                        V                O                            =                            ⁢                                                V                  S                                -                                  V                  F                                                                                                        =                            ⁢                                                                    V                    AUX                                    ×                                                            N                      S                                                              N                      A                                                                      -                                  V                  F                                                                                                        =                            ⁢                                                                    V                    FB                                    ×                                                                                    R                        ⁢                                                                                                  ⁢                        1                                            +                                              R                        ⁢                                                                                                  ⁢                        0                                                                                    R                      ⁢                                                                                          ⁢                      1                                                        ×                                                            N                      S                                                              N                      A                                                                      -                                  V                  F                                                                                        (        1        )            
Wherein, VFB is FB node voltage of controller 70, R0 and R1 are voltage divider resistors at FB node. NS and NA are turns of secondary-side output winding and auxiliary winding of the transformer 10, respectively. VF is forward voltage drop on the secondary rectifier 40. Controller 70 determines the output voltage according to voltage VFB sampled at FB node, so as to adjust the PWM duty cycle and maintain the output voltage at a certain predefined value. Referring equation 1, we found output voltage precision is mainly determined by inner error VFB generated in the controller 70 and periphery circuit error comprising resistor proportion, turns radio of the transformer and forward voltage drop on the secondary rectifier. Inner error generated in the controller should be reduced as much as possible to guarantee high yield rate.
FIG. 3 illustrates the functional block diagram of the controller in the prior-art primary-side feedback controlled AC/DC converter shown in FIG. 1. The shown controller comprises a sample hold module 100, an error amplifier 101, a slop voltage waveform generator 102, a comparator 103, a peak current comparator 104, a flip-flop 105 and a driving circuit 106. FIG. 4 illustrates the following relationship between the output voltage VEA of the error amplifier 101 and the switching cycle of the primary switch 20 of the primary-side feedback controlled AC/DC converter:T(VEA)=fT(VEA)  (2)
Wherein, fT( ) is the function determined by the stability and output precision of the primary-side feedback controlled AC/DC converter. As shown in FIG. 4, operation duration T is a monotone increasing function of output voltage VEA from the error amplifier 101, that is, the larger the output voltage VEA from the error amplifier 101, the longer the operation duration of the converter. Due to the switching frequency of the primary switch 20 f=1/T, the switching frequency f is a monotone decreasing function of output voltage VEA from the error amplifier 101, that is, the larger the output voltage VEA from the error amplifier 101, the smaller the switching frequency f.
FIG. 5 illustrates the circuit diagram of the prior-art error amplifier. Consisting of a transconductance amplifier 10 and a direct current resistor 11, the error amplifier has an output voltage gain as follows:A=gm×RO  (3)
Once the primary-side feedback controlled AC/DC converter is stable, it operates at discontinuous current mode, and output power from the transformer 10 is listed as follows:PT=η(½LPIP2f)   (4)
Wherein, η is power transform efficiency of the primary-side feedback controlled AC/DC converter, LP is the primary inductance of the transformer 10, IP is the peak current of inductor LP, f is the switching frequency. Normally, η, IP·LP all can be considered as constants.
The output power is listed as follows:POUT=VO×IO  (5)
Wherein, VO is output voltage, IO is output current. The output voltage VO decreases when POUT>PT, increases when POUT<PT and maintains constant when POUT=PT.
When the controller shown in FIG. 3 operates, sample hold module 100 obtains voltage VH from FB node. The controller operates in constant current output mode, if VH<VREF1, and operates in constant voltage output mode if VH>VREF1. Then, the output voltage of the error amplifier in FIG. 5 is listed as follows:VEA=A×(VH−VREF1)+VDC  (6)
Wherein, VDC is a fixed bias voltage, A is the voltage gain of the error amplifier. The output voltage as well as the VH and VEA increases when the output current decreases. The controller reduces the switching frequency, and the output power from the transformer decreases, stopping the increase of the output voltage. Similarly, the output voltage decreases when the output current increases. The controller raises the switching frequency, and the output power from the transformer increases, stopping the decrease of the output voltage. Supposing that the gain of the error amplifier is big enough, then the output voltage can be maintained at the voltage determined by following equation.
                              V          O                =                                            V                              REF                ⁢                                                                  ⁢                1                                      ×                                                            R                  ⁢                                                                          ⁢                  0                                +                                  R                  ⁢                                                                          ⁢                  1                                                            R                ⁢                                                                  ⁢                1                                      ×                                          N                S                                            N                A                                              -                      V            F                                              (        7        )            
However, due to the stability of the controller, the voltage gain of the error amplifier in FIG. 5 is normally defined at about 40 times, and the slop voltage changes from 1V-5V at different output loads, which means the primary switch 20 enjoys a high switching frequency and VEA approaches 1V when outputting a rated full load current, and the primary switch 20 enjoys a low switching frequency and VEA approaches 5V when outputting an empty load current. As the voltage change of VEA is 4V, the equivalent input change is:
                              Δ          ⁢                                          ⁢                      V                          REF              ⁢                                                          ⁢              1                                      =                                            Δ              ⁢                                                          ⁢                              V                EA                                      A                    =                                    4              40                        =                          0.1              ⁢                                                          ⁢              V                                                          (        8        )            
In such a way, huge voltage error of output voltage generates between a full load and an empty load, which results in low output voltage precision of primary-side feedback controlled AC/DC converter.